1. Field of the Invention
The present invention relates to a honeycomb filter and a method of manufacturing the honeycomb filter.
2. Description of the Related Art
Honeycomb filters are generally known in the art. The honeycomb filter is used to remove particulate matter (PM) from PM-containing exhaust emission discharged from a combustion device, such as an engine, and release the PM-free exhaust emission into the atmosphere. The honeycomb filter is structured to have a large number of cells parted by porous partition walls. The large number of cells include inlet opening cells with open inlets and outlets sealed with outlet sealing members and outlet opening cells with inlets sealed with inlet sealing members and open outlets. The inlet opening cells and the outlet opening cells are arranged alternately to be adjacent to each other. The PM-containing exhaust emission from the combustion device flows via the open inlets of the inlet opening cells included in the honeycomb filter into the respective inlet opening cells, passes through the porous partition walls, flows into the adjoining outlet opening cells adjacent to the respective inlet opening cells, and is released via the open outlets of the outlet opening cells to the atmosphere. The PM is trapped by the porous partition walls, as the PM-containing exhaust emission penetrates from the inlet opening cells through the porous partition walls and flows into the outlet opening cells. The exhaust emission flowing into the outlet opening cells is accordingly the PM-free clean emission.
When a prior art honeycomb filter in an unused condition is newly used to trap the PM, a pressure drop may abruptly rise immediately after a start of the PM trapping action. This may damage and deteriorate the performance of the combustion device. On a start of trapping the PM by the porous partition walls in the unused condition, depth filtration occurs concurrently with surface filtration. In the depth filtration, the PM invading the pores of the porous partition walls is trapped by the inside of the porous partition walls. In the surface filtration, the PM is trapped by the surface of the porous partition walls. After accumulation of the PM on the surface of the porous partition walls, cake filtration occurs. In the case filtration, the PM sedimentary layers have a filter function. In the initial phase of depth filtration, the PM is gradually accumulated in the pores of the porous partition walls. Such accumulation of the PM lowers the substantive porosity of the porous partition walls immediately after a start of the PM-trapping action and thereby causes an abrupt increase of the pressure drop.
In one proposed structure of the honeycomb filter disclosed in Utility Model Registration No. 2607898, a surface layer having smaller pores than the pores of the porous partition wall is formed on the inner surface of each of the inlet opening cells. This proposed structure enables the PM contained in the exhaust emission to be trapped by the surface of the surface layer even at the beginning of the PM-trapping action and accordingly prevents the PM from invading into the pores of the porous partition walls. This PM-trapping action does not lower the substantive porosity of the porous partition walls and prevents an abrupt increase of the pressure drop discussed above.